Electrocardiography (ECG) measures the electrical activity of the heart. It depicts the rate and the regularity of heartbeat as well as the presence of cardiac diseases or damage, arrhythmias etc. The ECG is one of the most important non-invasive diagnostic tools available to the cardiologists. It is measured by placing electrodes on the chest of the patient and measuring the bioelectrical potentials produced by the heart. Electrodes attached to the patient are connected by leads to an ECG monitor for further signal processing.
Standard methods for obtaining an ECG from a subject are 3 lead, 5 lead, 12 lead or a 15 lead ECG. 3 lead and 5 lead ECG are commonly used for routine ECG monitoring at hospitals. The 12 lead ECG recording provides much more information e.g. about possible cardiac ischemia than is obtainable from the 3 or 5 lead ECG. Therefore, the 12 lead ECG is the most common of these methods and thus often referred to as the “standard 12 lead ECG”. The 5 lead measurement provides seven signals: the limb leads I, II, III, aVR, aVL, aVF and one precordial lead e.g. V5. This measurement can be done by placing one electrode to each of the patient's four limbs at the wrists and ankles and one precordial electrode to the patient's chest. The limb electrodes are referred to as left arm (LA), right arm (RA), left leg (LL), and right leg (RL). For a standard 12 lead ECG, ten electrodes are attached to a patient's body in a manner described in FIG. 1. As shown in FIG. 1, six electrodes are attached in standard positions on the chest around the heart. The standard 12 lead ECG thus provides information from the frontal plane from limb leads I, II, III, aVR, aVL and aVF and from the horizontal plane from precordial leads V1, V2, V3, V4, V5 and V6. As is commonly known in the art, the ten electrodes are connected via lead wires and resistor networks to amplifiers to record twelve separate ECG channels or leads.
The frontal leads are obtained with various permutations of the LA, RA, and LL electrodes, with the RL electrode serving as an electrical ground. The frontal leads are comprised of the potential between two of the limb electrodes: lead I corresponds to the potential between LA and RA, lead II corresponds to the potential between LL and RA, and lead III corresponds to the potential between LL and LA. Leads aVR, aVL, and aVF, referred to as the augmented leads, are comprised of the potential between one electrode and a reference input, the reference input being the average of two electrodes. For example, lead aVF is the signal between LL and a reference input, where the reference input is the average of the potentials at electrodes RA and LA.
The horizontal leads V1-V6 are obtained with various permutations of the six electrodes attached to the patient's chest, in addition to three of the four limb electrodes. Each of the six horizontal leads is comprised of the signal between the potential at the particular electrode placed on the patient's chest and the potential at Wilson's central terminal. Wilson's central terminal refers to the average potential between the RA, LA, and LL electrodes shown simplified in FIG. 2. The three limb electrodes are connected through equal valued resistors to a common node and the voltage at this node, the Wilson central terminal, is the average of the voltages at each electrode. Each of the leads V1-V6 is compared to Wilson terminal, for example, lead V1 is the signal between electrode V1 and Wilson's central terminal.
It is not always clear when beginning the treatment how close monitoring will be needed in the future. The patient may go under a preliminary examination and a 3 or 5 lead ECG may be applied. Sometimes further examinations are needed e.g. for eliminating certain illnesses that can be detected by recording 12 lead ECG. Changing from 5 lead ECG to 12 lead ECG may be time consuming and difficult. Changing the electrode set and detaching and reattaching the electrodes can take considerable time, particularly if carried out by a non-specialist. This problem could be solved by always using the 12 lead or the 15 lead ECG electrode set and cable and only using the electrodes that are needed for that particular measurement. However, the 12 lead and the 15 lead ECG measurement cables are typically thick and long which makes the nursing staff favor the smaller and more convenient 5 lead ECG measurement electrode set. It is not desirable to have heavy wiring and plurality of unnecessary electrodes if the measurement accuracy of a 12 lead ECG is not needed.
In addition, if an EEG signal is to measured during examination, it will be a burden to the nursing staff to bring a separate EEG device for the measurement. The examination room or the operating room will get crowded if too many separate measuring equipment are needed.
In attempt to solve these problems, several solutions have been developed.
WO 01/06923 describes a system for ECG monitoring which enables the change between measuring 5 lead ECG and 12 lead ECG. In the measurement system described in the publication the 5-lead ECG is measured by connecting measuring electrodes by signal leads connected to a first connector element in a collecting connector. A second connector element of the collecting connector acts then as a shielding ground connection for the leads. A 12-lead ECG signal can be acquired by connecting additional measurement electrodes by leads to the second connector element of the collecting connector. The measurement signals are passed from the collecting connector to an amplifier unit and to the ECG apparatus for signal processing. The problem with the method described in the publication is that the lead wires when measuring 12 lead ECG will not be shielded and isolation of the system will be needed to ensure complete patient safety.
Publication EP 1221299 describes a method and an apparatus for generating a 12 lead ECG from fewer than ten electrodes. The method and apparatus of the publication generates a plurality of leads that are the same as the leads that would be generated from the electrical signals of a standard ECG. In the most preferred embodiment of the publication, electrodes V2, V3, V4, and V6 are not attached to the patient. Thus, leads V2, V3, V4, and V6 corresponding to the omitted electrodes must be derived mathematically by using multiple-linear regression. Eight of the leads, namely leads I, II, III, aVR, aVL, aVF, V1, and V5, are generated in the same manner as in a standard ten-electrode, twelve-lead ECG. In general, the four missing leads are calculated by using multiple-linear regression based on the relationship between the available leads and a data set of previously acquired ECGs. Although enabling the change between 5 lead ECG and 12 lead ECG, reducing the number of electrodes and calculating the signal for the missing leads may bring uncertainty in the ECG signal.
As is evident from the above, prior art solutions have several limitations and disadvantages.